Short fibers made of a thermoplastic resin, such as a polypropylene resin, and having a fiber length of about 1 to 20 mm are used commonly as reinforcing fibers for reinforcing various hardening materials, in particular, hydraulic materials such as concrete and mortar. In order to make use of their reinforcing abilities, the reinforcing fibers need to be difficult to remove from a material forming a cured article. For this reason, it has been proposed, for example, to mix the polypropylene resin with other resin to improve the fibers' affinity for the material forming a cured article or to form the fibers such that they have an oddly shaped cross section to enhance physical bonding between the fibers and cement.
For example, Patent Document 1 discloses cement reinforcing polypropylene fibers having a fiber strength of 6 g/denier or more. The fibers are highly crystalline polypropylene fibers having a Q-value of less than 5, a boiling n-heptane insoluble matter (HI) of 97<HI<100, and an isotactic pentad fraction (IPF) of 94≦IPF<100. A calcium carbonate fine powder is blended into at least a polypropylene resin that forms the surface layer of the fibers, and alkali metal salt of alkyl phosphate is adhered onto the fiber surface. Further, Patent Document 2 discloses cement reinforcing fibers having a cross section with three to six projections. An end of each projection has unevenness along the fiber length direction.